1. Field of the Invention
The present invention relates to an image-forming apparatus adopting an electrophotographic method, which develops a latent image formed on a photoconductive member by utilizing a laser beam.
2. Discussion of the Related Art
Digital electrophotographic methods have been widely employed as recording methods to provide an image of high quality at high speed. An area modulation method using a dot screen or line screen has been conventionally employed as a method of recording a halftone image for an image-forming apparatus adopting the electrophotographic method such as described above.
The area modulation method has a relatively simple algorithm which can be implemented at low cost. However, the basic construction of the area modulation method represents gradations of gray by repetition of white portions of paper and colored portions with toner or the like; therefore the repetition pattern of the white and colored portions is apt to be seen as image noise by the observer and cannot provide high image quality.
To obtain an image of high quality and of high resolution, an attempt has been made to combine a plurality of laser beams into one beam of extremely small diameter to form a latent image on a photoconductive member therewith, but the problem occurs that the design of the optical system is extremely difficult and the system is necessarily bulky.
On the other hand, as a method for recording a halftone image without employing the area modulation method, the method of modulating the light beam intensity corresponding to the image density required, referred to as the intensity modulation method is also known. According to this method, image noise caused by the repetition of white and colored portions in the process of image formation by the area modulation method does not occur.
In the intensity modulation method, an ultrasonic light modulator may be used or a method of varying the amount of current supplied to a semiconductor laser may be adopted to modulate the intensity of the light beam. However, an ultrasonic light modulator is expensive and intricate, and besides, high precision is required in installation. For this reason, the method of changing the amount of current supplied to a semiconductor laser is ordinarily used.
However, the semiconductor laser has a property that a small change in the input current causes a large change in the radiation output as shown in FIG. 6, and accordingly the problem occurs that a small change in the amount of input current supplied to the semiconductor laser corresponding to density data cannot provide an image which represents halftones adequately.
Many different methods including extremely stable power supply circuits have been proposed to overcome the above-mentioned problem, but it is very difficult to construct a stable power supply circuit, and besides, the circuit is necessarily bulky. Moreover, semiconductor lasers are highly dependent on environmental factors; therefore various compensating circuits, such as a circuit correcting for changes in temperature are required, thus leading to high cost and a bulky apparatus.
Japanese Patent Application Unexamined Publication No. Sho. 60-208162 (1985) proposes an image formation apparatus employing the intensity modulation method where the number of bits in the light intensity control signal is larger than that of the original image signal, so that an original image may-be reproduced with higher fidelity. However, there is an upper limit to the number of bits in the light intensity control signal (normally 8 bits) because of the limit to the data memory capacity per one picture element. To make the number of bits in the light intensity control signal larger than that of the original image signal, reducing of the number of bits in the original image signal to 6 bits for example, is required: therefore number of gradations of gray is reduced, thus resulting in difficulty in reproducing the gray-scale image with high fidelity.
In Japanese Patent Application Unexamined Publication No. Sho. 56-91228 (1981), the input density signal is modulated to generate a pulse signal for controlling a clock pulse signal of high frequency and applying the controlled clock pulse to a semiconductor laser, thus recording an image which includes some tens or more of halftone levels. However, this method is based on the area modulation method, and accordingly no solution is presented to the problem of image noise in the area modulation method caused by the repetitive pattern of paper white portions and colored portions.
There is another problem in adopting the intensity modulation method, in that the graininess of an image formed by this method is inferior to that of an image formed by the area modulation method even though toner of the same particle diameter is used in both cases. That is, the intensity modulation method is inferior to the area modulation method in smoothness of the formed image. The reason is as follows: it is known that the relation between density and graininess is roughly as shown in FIG. 7, wherein the parameter is the toner particle diameter. Every dot in a dot screen or every rectangle in a line screen is formed at a relatively high density which is predetermined, and the other low density portions are the bare surface of the recording medium. In other words, only the density ranges indicated as A and B in the figure are used in the area modulation method, resulting in comparatively good graininess characteristics even when toner of relatively large particle diameter is used.
On the other handy the intensity modulation method employs the whole density range shown in FIG. 7, and therefore the graininess of the image formed by this method is worse than that of an image formed by the area modulation method, particularly in areas of medium density.